A number of systems have been proposed for use as identification taggants, with an extensive body of work investigating methods for tagging explosives.
With respect to ammunition, a system has been proposed and tested wherein the addition of rare-earth elements to ammunition enhanced the delectability of gunshot residue by giving it an unambiguous composition due to incorporation of elements which are easily detected by neutron activation (Bryan et al., 1966). This method was only intended to provide a positive indication of the presence of gunshot residue. It was neither capable of encoding a usefully large number of identification codes, nor was any attempt made to encode any identification information in the taggants.